1. Technical Field
The present invention relates to an amorphous oxide semiconductor material, a field-effect transistor, and a display device.
2. Related Art
Field-effect transistors are widely used as unit elements of integrated circuits for semiconductor memories, high-frequency signal amplifying elements, and elements for driving display elements such as liquid crystal display elements, and transistors that have been made into a thin film are called thin-film transistors (TFT). Additionally, silicon TFTs having an active layer comprising amorphous silicon that is formable in a large area are used in flat panel displays.
In recent years, flat panel display technologies (large-screen, thin, lightweight) have undergone remarkable progress, and attempts to apply those flat panel display technologies to flexible substrates (displays that are lightweight and bend) have been made towards making them even thinner and more lightweight. On the other hand, silicon TFTs, which are currently the mainstream of TFTs that drive displays, have a high manufacturing temperature and it is difficult to form silicon TFTs on flexible substrates.
Amidst this background, oxide semiconductors are currently garnering attention as active layers of TFTs. In particular, In—Ga—Zn—O (hereinafter abbreviated as IGZO) reported by Hosono et al. of the Tokyo Institute of Technology shows promise as a material for new TFTs because it is formable at room temperature and exhibits excellent characteristics as a semiconductor even when amorphous, and studies on IGZO are being extremely vigorously carried out (see non-patent documents 1 and 2 listed below).
Among these studies, in relation to the optical properties of amorphous IGZO, when a TFT having an active layer comprising amorphous IGZO is applied to a display device such as an organic EL display, sometimes that active layer is irradiated with light, so as described in non-patent documents 3 to 6 listed below, many results of studies relating to the light irradiation properties of amorphous IGZO have been reported.
In non-patent documents 3 and 4, reports are made in relation to TFT properties when a TFT whose gate electrode comprises n-type Si, whose gate insulating film comprises a thermally-oxidized film, whose source and drain electrodes comprise Al and whose active layer comprises amorphous IGZO (In:Ga:Zn=1:1:1) has been irradiated with monochromatic light in a dark place.
Specifically, it can be confirmed that, when the irradiation amount is made constant and the wavelength of the monochromatic light is changed, the TFT properties virtually do not change (the drain current shifts slightly) when the TFT is irradiated with 460 nm≦λ≦660 nm monochromatic light, but when the TFT is irradiated with λ≦420 nm monochromatic light, the TFT properties greatly change (the threshold voltage Vth undergoes a 7 V negative shift when the TFT is irradiated with λ=365 nm (3.4 eV) monochromatic light). Further, it can be confirmed that, also when the monochromatic light wavelength is fixed (λ=420 nm) and the irradiation intensity is changed, changes in the TFT properties (negative shift in Vth) also become greater as the irradiation intensity increases. Moreover, it is understood that, in addition to Vth, the mobility, the sub-threshold swing (S value) and the off-state current also greatly change and IGZO is affected by the visible light short-wavelength region.
In non-patent documents 3 and 4, it is written that it is possible to drive an organic EL element without using a light blocking film because the transparent wavelength peak of a blue color filter and the wavelength peak of blue light of an organic electroluminescent element (organic EL element) are 450 nm. However, considering that a blue color filter passes about 70% of 400 nm light, that the skirt of the emission spectrum of blue light of an organic EL element continues to 420 nm, and that, thinking about the fabrication of a transparent device, the device is exposed to sunlight, it has not been shown that a TFT having IGZO fabricated by these documents as its active layer is sufficient.
Similarly, in non-patent documents 5 and 6 also, results of studies with respect to the light irradiation properties of amorphous IGZO are reported.
Specifically, first, polycrystalline IGZO where In:Ga:Zn=1:1:1 is used as a target to form an amorphous IGZO film (In:Ga:Zn=1.01:1.00:0.76). A TFT has, from its substrate side, the configuration of glass/Mo (gate electrode)/SiO2 or SiNx:H (gate insulating film)/IGZO (active layer)/SiO2/SiN (passivation layer)/Mo (source and drain electrodes). It can be confirmed that, by irradiating this TFT with λ≦440 nm monochromatic light, the TFT properties change (negative shift in Vth). Moreover, it can also be confirmed that the amount of change in the TFT properties becomes greater as the amount of time in which the TFT is irradiated with the monochromatic light is made longer. Further, by administering a heat treatment at 120° C., there is obtained the result that the TFT properties recover such that the same properties as the initial properties are obtained.
As a conclusion of the results of the studies above, it is understood that, when an active layer comprising amorphous IGZO of a TFT is irradiated with visible light, the TFT properties (an increase in the off-state current, a shift in Vth, etc.) greatly change with respect to light in the visible light short-wavelength region (light whose wavelength is in the neighborhood of 400 nm to 420 nm). Additionally, these changes in properties have a great affect on stable operation at the time of TFT driving.
Thus, when a TFT using amorphous IGZO for its active layer is used as a TFT for driving a display element, in order to allow the TFT to stably operate, in patent document 1, light blocking means with respect to the amorphous IGZO is employed. Further, in patent document 2, a structure that does not expose the amorphous IGZO to blue light (light in the visible light short-wavelength region) is employed.
However, in the inventions of patent documents 1 and 2, the purpose is to not allow the amorphous IGZO serving as the active layer to be irradiated with light, and there is no description in relation to improving the light irradiation properties of the amorphous IGZO itself.
Here, it is conceivable to control the composition ratio of In, Ga and Zn in order to improve the light irradiation properties of the amorphous IGZO. To that end, materials that limit the composition ratio of the amorphous IGZO to particular ranges have also been proposed (e.g., see patent documents 3 to 6 listed below).
In patent document 3, there is reported an amorphous oxide represented by Znx[Al, Ga]yInzO(x+3y/2+3z/2):[ratio x/y is in the range of 0.2 to 2, and ratio z/y is in the range of 0.4 to 1.4]. As an example, there is shown a fabrication example in the neighborhood of In:Ga:Zn=1:1:1.
However, the purpose is to use the amorphous IGZO as a transparent conductive film, that is, as an electrode, and the purpose is not to use the amorphous IGZO as a semiconductor. Further, a carrier concentration of a considerably high density is supposed because the electron concentration is equal to or greater than 1×1018 to 1022/cm3 (1019/cm3 even in relation to the example). When this amorphous oxide is used as an active layer of a transistor, it is conceivable that the realization of normally-off is extremely difficult.
In patent document 4, there is reported an amorphous oxide semiconductor material of a composition where the atomic composition ratio represented by In/(In+Zn) is from 35 at. % to 55 at. % and where the atomic composition ratio represented by Ga/(In+Ga+Zn) is equal to or less than 30 at. %.
However, no experimental data relating to light absorption are shown as examples.
In patent document 5, there is reported a TFT having as its active layer an amorphous IGZO capable of passing a constant current stably for a long time. This document has a graph showing changes in the TFT properties (Vg-Id characteristics) at the time of light irradiation.
However, in patent document 5, there is fabricated a TFT having no property changes with respect to light irradiation in an In-rich, Ga-poor and Zn-poor region, and there is no description suggesting photostability in a Ga-rich region.
Further, because there is no detailed description relating to the light source, it is unclear which wavelength region of light is affecting the TFT, and it is also unclear whether or not there is photostability with respect to the visible light short-wavelength region and particularly the visible light short-wavelength region of 400 to 420 nm. Consequently, when there is a defect or the like in the IGZO, sometimes the IGZO is affected even by light in the visible light short-wavelength region, so there is also the potential to fabricate a TFT having no property changes with respect to light irradiation in the visible light short-wavelength region. Further, there is also the potential to manufacture a TFT having no property changes with respect to light irradiation in the visible light short-wavelength region outside the range of 400 to 420 nm.
Moreover, in the amorphous IGZO configuring the active layer of each TFT, the composition ratios of In, Ga and Zn differ because of the result of inductively coupled plasma (ICP), but in the end it is unclear which composition amount is affecting the improvement of photostability.
Moreover still, the same target is used to fabricate the amorphous IGZO, so considering ICP error (±0.2), plural TFTs having active layers comprising amorphous IGZO of substantially the same composition are fabricated, but the TFT properties with respect to light irradiation of each TFT greatly differ, and reproducibility is considered to be low.
In patent document 6, there is reported a field-effect transistor having for its active layer an amorphous oxide semiconductor that includes the element In, the element Zn and element X (the element Ga is included as one candidate for element X) and satisfies In/(In+Zn+X)=0.200 to 0.600 and Zn/(In+Zn+X)=0.200 to 0.800. In the examples, there are described the electrical properties of a TFT having for its active layer IGZO in which the Ga concentration has been changed, but there are no data relating to the optical properties or results suggesting photostability in a Ga-rich region. Additionally, a composition range is claimed including also a Ga-rich region with good photostability, but according to the specification, it is necessary for the transistor to take a structure that blocks a semiconducting phase, so it is thought that the optical properties are not good. The field-effect transistor also differs from the present invention in that the film thickness is from 1 nm to 15 nm and extremely thin.
Non-Patent Document 1: Science, 300 (2003), pp. 1269-1272
Non-Patent Document 2: Nature, 432 (2004), pp. 488-492
Non-Patent Document 3: Journal of Information Display, 9 (2008), pp. 21-29
Non-Patent Document 4: SID08 Digest (2008), pp. 1215-1218
Non-Patent Document 5: Japanese Journal of Applied Physics, 48 (2009), pp. 03B018-1-03B018-5
Non-Patent Document 6: “Instability of Amorphous IGZO TFTs under Light Illumination,” The 15th Int. Workshop on Active-Matrix Flatpanel Display and Devices [AM-FPD08]˜TFT Technologies and FPD Materials˜(Jul. 2-4, 2008, Tokyo, Japan)
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2007-115902
Patent Document 2: JP-A No. 2007-250984
Patent Document 3: Japanese Patent No. 4,170,454
Patent Document 4: JP-A No. 2007-281409
Patent Document 5: U.S. Patent Application Publication No. 2007/0252147
Patent Document 6: JP-A No. 2009-253204